Saturday, September 6, 2014

IR Shot Detection Details

I haven't provided much detail on the beam-breaker shot detection setup in the AVI-TAR prototype so this post will be dedicated to that. What I started out with is a bit of a kludge but I'm working on alternatives that I'll explain at the end.

The Prototype

In order to quickly put the prototype together I had to come up with a shot detector that didn't require a lot of fabrication or special parts. I needed to put an IR emitter/detector pair somewhere in the path of the BB as it traveled past the hop-up and down the barrel. BB movement detection in the feed tube from the magazine, before the air-nozzle, is pretty difficult since the BBs are stacked in the tube "shoulder to shoulder". There are hop-up assemblies available that use that detection method, but only to signal an empty magazine condition.

So...where to put the detector. My first attempt, used in the working prototype, is this:

About 6" down from the hop-up I machined two slots into the outer barrel and cross-drilled so that I can see the inner barrel. If you'll note the notch at the top of the barrel closer to the hop-up, that's where a mounting plate notches into the barrel and fastens down into the lower barrel bed in the body of the AEG. I have to allow clearance for that.

The IR detector is hot-glued to the outer barrel and I fortified it with a little tape wrap (removed here for the picture).

The inner barrel is cross-drilled and a very thin clear tape is applied over the holes once the barrel is de-burred inside and out:


After the drilling and performed and the barrel assembly is back together, I can use the AVI-TAR system to verify its alignment via a testing tool I wrote into the main menu. It simply tells me if the IR detector is "on" or "off". A cleaning rod shoved down the barrel helps verify proper operation. Overall, a pretty simple operation.

Improvements?

Obviously, hot-glue and tape are not a good solution for mounting the IR detector. I also am not fond of machining the barrels. The problem is that there are only a few places to properly put positive shot detection sensors:
  • In the hop-up chamber. It can be very difficult since the interior of the hop-up chamber contains the rubber bucking and the barrel, neither of which are transparent to light. There are clear buckings available so it may be that a new hop-up chamber could be engineered to meet my needs. It's pretty complex part and would need to be the same size as the current hop-up chamber to fit a stock TAR.
  • In a mock-suppressor. There is actually a new system out there that does just this (and uses 2 detectors to give you a chrono feature). My problem with such an assembly is that it needs to be connected to the controller for power and signal. Any kind of cabling harness would not look good, would be a weak spot on the gun and would be a pain to work with when you did maintenance.  The alternative is a wireless system. This requires more complex electronics and a separate power source for the detector assembly. Oh, no...more batteries to manage.
  • Outer-barrel extension. This is a decent possibility if engineered properly. Unlike a suppressor, this would allow users to put whatever barrel-end on their TAR, be it the standard flash hider or any suppressor they prefer.
  • In the existing barrel assembly. This requires machining of parts but results in a clean look since it would be inside the gun body. The machining isn't a lot of work but it would not be "parts swap" operation unless one could obtain pre-worked barrel assemblies. The detector would be mounted in a "clamp-on" assembly that fits into the machined outer barrel. This would stand up to the rigors of use and would be easy to maintain.

That wraps up this post. See you on the field, maybe.